1. Field of the Invention
The present invention relates to a band pass filter device, more specifically, relates to a filter device including an LC parallel resonance circuit and an elastic wave resonator.
2. Description of the Related Art
Various existing filter devices have been proposed as band pass filters for mobile communication devices and the like. A pass band of the filter device of this type is required to be wide in many cases.
Japanese Unexamined Patent Application Publication No. 2011-234191 discloses a band pass filter device capable of widening a pass band. In the band pass filter device as disclosed in Japanese Unexamined Patent Application Publication No. 2011-234191, a high pass filter circuit and a low pass filter circuit are connected in series. The high pass filter circuit includes a high pass surface acoustic wave resonator and a high pass inductance element that is connected in parallel with the high pass surface acoustic wave resonator. On the other hand, the low pass filter circuit includes a first surface acoustic wave resonator that is arranged on a serial arm and one end of which is connected to an end portion of the high-pass filter circuit.
A second surface acoustic wave resonator is connected between the one end of the first surface acoustic wave resonator and a ground potential and a low pass inductance element is connected in parallel with the second elastic wave resonator. Further, the second surface acoustic wave resonator is also connected between the other end of the above-mentioned first surface acoustic wave resonator and the ground potential. A low pass inductance element is also connected in parallel with the second surface acoustic wave resonator.
Japanese Unexamined Patent Application Publication No. 2007-202136 discloses a surface acoustic wave (SAW) filter having a wide pass band in a digital terrestrial television (TV) radio wave band. In the SAW filter as disclosed in Japanese Unexamined Patent Application Publication No. 2007-202136, two serial arm resonators are provided on a serial arm connecting an input terminal and an output terminal. Inductance for shifting resonant frequency/anti-resonant frequency is connected between the two serial arm resonators in series to the two serial arm resonators.
Further, a parallel arm resonator is arranged on a parallel arm connecting the input terminal and a ground potential. An inductance element is connected in parallel with the parallel arm resonator. In the same manner, a parallel arm resonator is also provided on a parallel arm connecting the output terminal and the ground potential. An inductance element is also connected in parallel with the parallel arm resonator.
The inductance element connected to each of the above-mentioned parallel arm resonators is provided for shifting a resonant frequency and an anti-resonant frequency by the parallel arm resonator.
In Japanese Unexamined Patent Application Publication No. 2011-234191 and Japanese Unexamined Patent Application Publication No. 2007-202136, as described above, the resonant frequency or the anti-resonant frequency of the surface acoustic wave resonator arranged on a line connecting the input terminal and the output terminal, in other words, the serial arm, or of the surface acoustic wave resonator arranged on the parallel arm connecting the line and the ground potential, can be adjusted. This adjustment widens the band, improves attenuation characteristics, and so on.
It has been, however, difficult to further widen the pass band with the band pass filter using the surface acoustic wave resonator. For example, in a ladder circuit, the resonant frequency of the parallel arm resonator and the anti-resonant frequency of the serial arm resonator form attenuation poles at the low-frequency region side and the high-frequency region side. Accordingly, it is sufficient that they are distanced from each other in order to widen the band. However, when a difference in frequency between the attenuation pole at the low-frequency region side and the attenuation pole at the high-frequency region side is made larger, there arises a problem that attenuation at the center of the pass band is increased. Accordingly, widening of the band has been limiting.
In addition, the ladder filter device also has a problem that attenuation in an attenuation band separated from the attenuation pole is not sufficiently large.